I wouldn't give Zyrtec to a child that young!! Ask your doctor his/her rationale for prescribing it.
According to the manufacturer, the safety and effectiveness of Zyrtec has NOT been checked on children under the age of 2 YEARS. Yet, your doc wants to give it to an 8 month old child???
I'd try using some Aveeno cream on her rash and see if that helps before medictaing her. Also, Aveeno makes a bath product that is good, too. Old-fashioned oatmeal baths are hard to beat, and they won't aggravate the skin.
Years ago, I had a rash on my legs they couldn't diagnose. The dermatologist I saw prescribed a compound made of Moisturel lotion and menthol. The pharmacist made it up, and it helped clear it up almost overnight. Simple, cheap and no side-effects to the body. What could be better? ((I always liked that this mainstream dermatologist believed in old-fashioned remedies!!))
Zyrtec is good, but it IS a powerful medicine. If she only has a rash, and no other symptoms (runny/stuffy nose, sniffles, teary eys), why medicate her entire system?? Try topical treatments first and see if they help.